JP4569049B2 - Mass spectrometer - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a mass spectrometer that ionizes a sample under a pressure close to atmospheric pressure, such as an electrospray mass spectrometer (ESI-MS) or an atmospheric pressure chemical ionization mass spectrometer (APCI-MS).
[0002]
[Prior art]
When a liquid sample separated by liquid chromatography (LC) or the like is introduced into a mass spectrometer (MS) and detected, an ionization interface such as electrospray method (ESI) or atmospheric pressure chemical ionization method (APCI) is used. The Each of these ionization methods ionizes a sample under a pressure that is relatively close to atmospheric pressure, and includes an ionization chamber that is in a high pressure state (ie, close to atmospheric pressure) and a very low pressure state (ie, a high vacuum level). In order to ensure a pressure difference with the mass spectrometry chamber in the state), one or a plurality of intermediate chambers are provided between the two chambers, and the degree of vacuum is gradually increased. .
[0003]
In such a configuration, in order to ensure a differential pressure between the ionization chamber and the subsequent intermediate chamber, or between the intermediate chamber and the subsequent intermediate chamber, a small ion passage hole (generally “orifice”) is formed at the top of the ionization chamber. Conical skimmers having a so called) are used. Further, in the intermediate chamber, means for assisting ion transport called an ion lens or an ion guide is used in order to efficiently transport ions to the subsequent stage.
[0004]
The main action of the ion lens and ion guide is to accelerate flying ions while converging flying ions by an electric field, and various shapes have been proposed. As one of them, a so-called multipole ion guide is known in which an even number of pole-shaped electrodes extending in the optical axis direction are arranged apart from each other around the ion optical axis. As a person has proposed in the invention described in Japanese Patent Application Laid-Open No. 2000-149865, the pole-like electrode can be replaced with an assembly of plate-like electrodes if it has an equivalent action. In such a multi-pole type ion guide, a voltage (in other words, a DC biased high frequency voltage) in which a high frequency voltage having a phase inverted to each other is superimposed on the same DC voltage is applied to the pole electrodes adjacent in the circumferential direction. Voltage) is applied. Due to the high-frequency electric field generated by this, ions introduced into the ion guide proceed while vibrating at a predetermined cycle, and are efficiently transported to the subsequent stage.
[0005]
[Problems to be solved by the invention]
In LC / MS, since most of the liquid sample is often a solvent, a large amount of low-mass ions due to the solvent are generated in the ionization chamber. Usually, the solvent passes through the LC column faster than the target component and reaches the ionization chamber, so that a large amount of low-mass ions resulting from this is introduced into the multipole ion guide through the skimmer orifice. become. Then, it affects the space charge in the ion guide and may adversely affect the passage of high-mass ions coming later. As a result, the transport efficiency of the target high-mass ions is deteriorated, and as a result, the analysis sensitivity may be lowered.
[0006]
The present invention has been made in order to solve the above-mentioned problems. The object of the present invention is to eliminate unwanted low-mass ions as much as possible, and to achieve the desired high mass (if multivalent ions, mass). The object is to provide a mass spectrometer equipped with an ion transport optical system that can efficiently transport ions to the subsequent stage.
[0007]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present invention includes a skimmer having a passage hole on an ion optical axis, and a plurality of poles arranged at a subsequent stage so as to surround the ion optical axis and to be separated from each other. An ion transport optical system including a multipole ion guide composed of electrodes, and by applying a voltage obtained by superimposing a DC voltage and a high frequency voltage to each pole electrode of the ion guide, In a mass spectrometer that transports ions that have passed through to the subsequent stage,
a) voltage adjusting means for adjusting at least one of a DC voltage applied to the ion guide and a DC voltage applied to the skimmer;
b) In order to create an electrostatic field in the space between the skimmer and the ion guide that excludes ions having a mass smaller than a predetermined mass , the voltage difference between the DC voltage applied to the ion guide and the skimmer is Control means for controlling the voltage adjusting means so as to have a predetermined value corresponding to the predetermined mass ;
It is characterized and Bei Etako a.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
Since the velocity v of the ion immediately after passing through the skimmer passage hole is almost independent of the mass m of the ion and is constant, the kinetic energy E of the ion is almost proportional to the mass m. The smaller the kinetic energy E, the more easily ions are affected by the electric field existing in the space between the skimmer and the ion guide, and the flight trajectory is likely to change. The mass spectrometer according to the present invention takes advantage of this fact and adjusts each DC voltage so as to appropriately set the voltage difference between the DC voltage applied to the skimmer and the DC voltage applied to the ion guide. An appropriate electrostatic field is generated so that ions having a mass smaller than a predetermined mass are not introduced into the ion guide, or the ion guide is not introduced within a receiving range capable of appropriately transporting ions to the subsequent stage. To do.
[0009]
Since the state of the electrostatic field depends on a direct current voltage difference between the skimmer and the ion guide, it is natural that the direct current voltage of the skimmer or the ion guide may be a ground potential.
[0010]
As an aspect of the present invention, an operator can input a predetermined value that is a target value of the voltage difference. As another aspect of the present invention, the operator may input a mass upper limit value of low mass ions to be excluded and automatically calculate a voltage difference according to the value.
[0011]
【The invention's effect】
According to the mass spectrometer of the present invention, low-mass ions caused by the solvent of the liquid sample can be eliminated before the ion guide, so that the space potential in the ion guide is affected by these low-mass ions. Therefore, it is possible to prevent the transport of high mass ions from being hindered. Therefore, the transport efficiency of high-mass ions that are the object of analysis is improved, and as a result, the analysis sensitivity can be improved.
[0012]
【Example】
Hereinafter, an embodiment of a mass spectrometer according to the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram of a main part of the ESI-MS according to the present embodiment.
[0013]
The MS includes an ionization chamber 10 in which a nozzle 11 connected to an outlet end of a liquid chromatograph column (not shown) is disposed, an analysis chamber 18 in which a quadrupole filter 19 and a detector 20 are disposed. In between, a first intermediate chamber 12 and a second intermediate chamber 15 are provided, each separated by a partition wall. A small-diameter desolvating pipe (heated capillary) 13 is provided between the ionization chamber 10 and the first intermediate chamber 12, and a small-diameter orifice 16a is formed at the apex between the first intermediate chamber 12 and the second intermediate chamber 15. Communicating only through a conical skimmer 16 having
[0014]
The inside of the ionization chamber 10 is almost at atmospheric pressure due to vaporized molecules of the sample liquid continuously supplied from the nozzle 11, and the inside of the first intermediate chamber 12 is evacuated to a low vacuum state of about 10 ² Pa by a rotary pump. The The inside of the second intermediate chamber 15 is evacuated to a medium vacuum state by a turbo molecular pump to about 10 ⁻¹ to 10 ⁻² Pa, and the inside of the analysis chamber 18 is also about 10 ⁻³ to 10 ⁻⁴ Pa by the turbo molecular pump. It is evacuated to a high vacuum state. That is, by increasing the degree of vacuum for each chamber from the ionization chamber 10 toward the analysis chamber 18, the inside of the analysis chamber 18 at the final stage is maintained in a high vacuum state.
[0015]
The liquid sample is sprayed into the ionization chamber 10 from the nozzle 11, and the sample molecules are ionized in the process of evaporating the solvent in the droplets. Fine droplets mixed with ions are drawn into the desolvation pipe 13 due to the differential pressure between the ionization chamber 10 and the first intermediate chamber 12, and the solvent evaporates in the process of passing through the desolvation pipe 13, and further ionization proceeds. . In the first intermediate chamber 12, opposed flat plate-shaped or ring-shaped lens electrodes 14 are provided, and the electric field formed by the lens electrodes 14 assists the drawing of ions through the desolvation pipe 13. The ions are converged in the vicinity of the orifice 16 a of the skimmer 16.
[0016]
The converged ions pass through the orifice 16a of the skimmer 16 so as to be drawn by the differential pressure between the first and second intermediate chambers 12 and 15, and are converged by the ion guide 17 to the analysis chamber 18 as will be described later. Sent. In the analysis chamber 18, only ions having a specific mass number (m / z) pass through the longitudinal space in the center of the quadrupole filter 19 and reach the detector 20 to be detected.
[0017]
2 is a schematic perspective view of the vicinity of the skimmer 16 and the ion guide 17 of this apparatus, and FIG. 3 is a detailed configuration diagram of the same part.
[0018]
As shown in FIG. 2, the ion guide 17 has a configuration in which a plurality of substantially cylindrical pole-shaped electrodes are spaced apart from each other so as to circumscribe a circle A having a predetermined diameter centered on the optical axis C. ing. In this example, the number of pole-shaped electrodes is four. However, the number of pole-shaped electrodes is not limited to this and may be an even number of four or more. Moreover, the structure which can be regarded as a pole-shaped electrode substantially as mentioned above may be sufficient.
[0019]
Each pole electrode constituting the ion guide 17 is connected to the ion guide voltage applying section 22 (however, only a part is shown in FIG. 3 to avoid complexity), and the pole-shaped electrodes adjacent in the circumferential direction In this case, a voltage in which high-frequency voltages whose phases are inverted to each other is superimposed on the same DC voltage V1 is applied. Due to the high-frequency electric field generated in the space surrounded by the pole-like electrodes by such applied voltage, the ions introduced into the space travel so as to cling to the ion optical axis C while vibrating at a predetermined period. On the other hand, the DC voltage V <b> 2 is applied from the skimmer voltage application unit 21 to the preceding stage skimmer 16. The skimmer voltage application unit 21 and the ion guide voltage application unit 22 perform a voltage application operation under the control of the control unit 23. The entity of the control unit 23 is a computer, and controls each unit of the MS in addition to the voltage application units 21 and 22 based on various parameters input from the operation unit 24.
[0020]
In the above configuration, as described above, ions are attracted by the differential pressure and pass through the orifice 16a of the skimmer 16, so that the ion velocity v immediately after passing through the orifice 16a is a substantially constant value regardless of its mass m. Become. This has been confirmed by our experiments. On the other hand, the kinetic energy E of the ions at that time is
E = (1/2) · m · v ²
Therefore, the kinetic energy E is proportional to the mass m. Therefore, ions having a larger mass m are introduced into the second intermediate chamber 15 through the orifice 16a with a larger kinetic energy E.
[0021]
When an electrostatic field is present in the space between the skimmer 16 and the ion guide 17, ions having smaller kinetic energy E are more easily affected by the electric field, and the flight trajectory is likely to deviate greatly from the ion optical axis C. For example, when a certain electrostatic field is formed, high-mass ions are introduced into the ion guide 17 with a flight trajectory as indicated by BH in FIG. 3, and are transported to the subsequent stage while vibrating in the internal space. . On the other hand, the low-mass ions are more strongly affected by the electrostatic field, and as shown by BL in FIG. 3, the DC potential difference between the skimmer 16 and the ion guide 17 cannot be overcome and can enter the ion guide 17. Can not.
[0022]
Since the state of the electrostatic field depends on the voltage difference ΔV (= | V2−V1 |) between the voltage V1 and the voltage V2, by passing through the skimmer 16 by adjusting this voltage difference ΔV, it is before the ion guide 17. The maximum mass of ions that diverge can be determined. In other words, when it is desired to diverge ions having a mass smaller than a certain mass, the voltage difference ΔV may be adjusted. In this apparatus, for example, the operator inputs the maximum mass of low-mass ions to be removed from the operation unit 24. The control unit 23 has a table (or calculation formula) indicating the correspondence between the mass and the voltage difference ΔV in advance, and calculates the voltage difference ΔV corresponding to the input. Then, the values of V1 and V2 are determined based on the voltage difference ΔV, and the voltage applying units 21 and 22 are controlled. Of course, various modes such as a configuration in which the operator inputs the voltage difference ΔV itself instead of inputting the maximum mass of ions to be excluded can be considered.
[0023]
According to this apparatus, the low-mass ions resulting from the solvent are not introduced into the ion guide 17, and the space charge in the ion guide 17 is not affected by the low-mass ions. Therefore, the high mass ions are appropriately converged and sent into the analysis chamber 18 without hindering the passage of the high mass ions introduced later.
[0024]
It should be noted that the above-described embodiment is merely an example of the present invention, and it is apparent that the present invention is encompassed by the present invention even when appropriate changes and modifications are made within the scope of the present invention.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a main part of a mass spectrometer according to an embodiment of the present invention.
FIG. 2 is a schematic perspective view in the vicinity of a skimmer and an ion guide of the mass spectrometer of the present embodiment.
FIG. 3 is a detailed configuration diagram in the vicinity of a skimmer and an ion guide of the mass spectrometer of the present embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Ionization chamber 11 ... Nozzle 12 ... 1st intermediate chamber 13 ... Desolvation pipe 14 ... Lens electrode 15 ... 2nd intermediate chamber 16 ... Skimmer 16a ... Orifice 17 ... Ion guide 18 ... Analysis chamber 19 ... Quadrupole filter 20 ... Detector 21 ... Skimmer voltage application unit 22 ... Ion guide voltage application unit 23 ... Control unit 24 ... Operation unit C ... Ion optical axis

Claims (1)

Ion transport including a skimmer having a passage hole on the ion optical axis and a multipole ion guide composed of a plurality of pole-shaped electrodes arranged separately from each other around the ion optical axis. In a mass spectrometer that includes an optical system and transports ions passing through a passage hole of the skimmer to a subsequent stage by applying a voltage obtained by superimposing a DC voltage and a high-frequency voltage to each pole electrode of the ion guide. And
a) voltage adjusting means for adjusting at least one of a DC voltage applied to the ion guide and a DC voltage applied to the skimmer;
b) In order to form an electrostatic field in the space between the skimmer and the ion guide that excludes ions having a mass smaller than a predetermined mass , the voltage difference between the DC voltage applied to the ion guide and the skimmer is Control means for controlling the voltage adjusting means so as to have a predetermined value corresponding to the predetermined mass ;
Mass spectrometer, wherein the Bei Etako a.

Images